Metal alloys have been employed in the dental industry for many, many years in the fabrication of crowns, bridges and other prosthetic appliances. In order to reduce the high cost involved in using gold and platinum in the manufacture of dental appliances, silver became a popular and inexpensive metal substitute. Furnaces employed in the preparation of dental restorations are typically used for a large variety of dental restorations irrespective of the alloy type. Although using the furnace for the heating of a variety of materials is considered an efficient utilization thereof, problems arise due to the contamination of the furnace interior with residue from the processing occurring therein.
One specific problem arises from silver and/or copper impurities remaining on the furnace interior after the firing of porcelain or silver containing alloys. It is speculated that silver impurities present in the furnace during the firing of ceramic material cause discoloration in the resultant porcelain material. This discoloration is also referred to as "greening" and is believed to occur when silver appears in glass as Ag.sup.0. Maintenance and cleaning of furnaces by scrubbing, purging and the like have helped to reduce the greening effect, but have not been either totally effective or completely efficient. One known method discussed in U.S. Pat. No. 4,405,300 to Lubowsky et al. uses carbon as an absorbent. Carbon is positioned in an empty, contaminated furnace and a firing cycle is completed in the furnace. The carbon absorbs contaminants in the furnace and thereby, the furnace is cleansed and purged of impurities. This method is effective, but is inefficient and inconvenient since the furnace and operator must partake in unproductive and unprofitable firing cycles. This purging process must be performed frequently to provide consistency in the quality of the ceramic restorations produced.
Another form of discoloration or "greening" in dental restorations can occur from the alloy coping itself when it is composed of silver, copper or some other alloy that may form reduced metal (Me.sup.0) clusters in overlay porcelain.
High fusion temperature porcelains having non-greening properties have been developed and are currently in use. These porcelains are typically utilized as body porcelains because they tend not to be translucent enough for the outer layers of the restorations. It is often necessary to increase the opacity of these porcelains to mask their sometimes speckled appearance.
There is a need to provide low fusing porcelain compositions that do not discolor when fired in contaminated furnaces or applied on silver or copper containing metal copings. It is advantageous that the firing furnace be used to its full potential. There is a need to provide low fusing porcelains which are transparent and do not discolor.